The invention relates to cutter assemblies for rotary drill bits for use in drilling or coring deep holes in subsurface formations.
The cutting assemblies are for use in rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and/or cooling the cutters.
Each cutter assembly comprises an elongate stud which is received in a socket in the surface of the bit body, the stud having mounted at one end thereof at least one preform cutting element. The preform cutting element may be of the kind comprising a tablet, often circular or part-circular, having a thin hard cutting layer of polycrystalline diamond bonded to a thicker, less hard backing layer, for example of tungsten carbide. However, preform cutting elements are also known which consist of a unitary body of thermally stable polycrystalline diamond.
The studs of this kind of cutting assembly are often in the form of a cylinder of circular cross-section. Studs of this form have the advantage that their simple geometry facilitates the formation of appropriate sockets in the bit body, whether such sockets are formed by machining in a steel bodied bit, or by mounting suitably shaped formers in the mould in the case where the bit body is formed from solid infiltrated matrix in a powder metallurgy process. In each case the simple cylindrical form of the required sockets not only means that they can be manufactured at low cost, but it also facilitates the dimensioning of the sockets so as to control, for example, the braze gap between each stud and its socket in cases where the stud is to be secured in the socket by brazing.
Cylindrical sockets of circular cross-section, however, have the disadvantage that the stud can adopt any rotational position in the socket. This means that it is then necessary to ensure that each stud is correctly rotationally orientated in its socket before it is secured in position. However, the loads to which a cutting assembly is subjected during drilling may often result in the application of a substantial torque to the stud and it is therefore sometimes found that such studs become rotationally displaced in their sockets in the course of use.
Furthermore, in order to provide adequate strength to the mounting of the cutter assemblies in the bit body, it is necessary to provide a certain minimum thickness of solid material between adjacent sockets along the whole of their lengths. Since rows of cutting assemblies are often disposed side by side along convexly curved portions of the bit body, the inner ends of the sockets are closer together than the outer ends and, consequently, it may often not be possible to arrange the cutting elements, on the projecting outer ends of the posts, as close together as is desirable.
Attempts have been made to overcome these problems by the use of studs which are non-circular in cross-section. For example, studs which are of generally rectangular or similar cross-section may be packed together more closely in the bit body than studs of circular cross-section and the orientation of the socket automatically fixes the orientation of the stud inserted in it. However, the corresponding non-circular sockets are difficult and costly to manufacture with the required accuracy. The present invention sets out to provide a new form of cutter assembly where the above mentioned problems are reduced and yet where the studs and corresponding sockets may still be manufactured to the required degree of accuracy at a comparatively low cost.